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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Atom-Photon Entanglement in a Cavity

Tatjana Wilk, S. C. Webster, Axel Kuhn, Gerhard Rempe

Published Jun 1, 2007• 0 citations
Publisher

Abstract

Creating entanglement between distant nodes of a quantum network is a major task in quantum information processing. Deterministic creation of atom-photon entanglement would be a big step towards this aim since photons can travel large distances without decoherence. Entanglement swapping between two such atom-photon pairs should be a feasible way to entangle two distant atoms and thus realise a quantum repeater. Here, we show a way to achieve entanglement in a strongly coupled atom-cavity system.

DOI
10.1109/cleoe-iqec.2007.4386757

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Creating entanglement between distant nodes of a quantum network is a major task in quantum information processing. Deterministic creation of atom-photon entanglement would be a big step towards this aim since photons can travel large distances without decoherence. Entanglement swapping between two such atom-photon pairs should be a feasible way to entangle two distant atoms and thus realise a quantum repeater. Here, we show a way to achieve entanglement in a strongly coupled atom-cavity system.
Atoms and photons are well-suited candidates for quantum information processing and quantum communication. While atoms can be used for storage and manipulation, photons are ideal carriers for the transfer of quantum information. Our group pursues the goal of creating a basic node of a quantum communication network by entangling two independently trapped87Rb atoms over a large distance. Besides its prospects in quantum communication (e.g. quantum repeater), this would also allow a loophole-free test of Bell's inequality which can help to conclusively answer the long-standing question about the completeness of the quantum theory. The key ingredient is atom-photon entanglement which will enable us to entangle two atoms by entanglement swapping. We plan to achieve a distance of 300m between two independent single atom traps.
We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90±2)%. The protocol works for any distance between cavity-coupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g., for entanglement swapping in a quantum repeater.
Efficient quantum repeaters are needed to combat photon losses in fibers in future quantum networks. Single atom coupled with photonic cavity offers a great platform for photon-atom gate. Here I propose a quantum repeater scheme with efficient entanglement generation and entanglement swapping based on photon-atom gates. It can be implemented with various types of atomic systems and requires much less experimental complexity compared to other repeater protocols. With current available experimental techniques and reasonable improvements, high entanglement distribution rates can be achieved. A multiplexing configuration of 10 single atoms in cavities, secret key rates in order of a few Hz to 100 s Hz can be achieved for communication distance of 1000 km. This proposal paves the way for the demonstration of an efficient entanglement distribution with quantum repeaters in the near future.
Quantum repeaters are vital ingredients for realization of long-distance quantum networks [1]. They can be realized by preparing nodes that can generate an entangled light-matter state, namely, a single photon entangled to a quantum memory. This allows for entangling two distant quantum memories, by propagating the single photons from each node to an intermediate place for Bell state measurements. A common approach for generating such entangled light-matter states is generating an entangled photon pair in a nonlinear source, and coupling one of the photons in the pair to a quantum memory for storage [2]. However, such an approach faces two main challenges: Photon-pair generation in nonlinear sources is a probabilistic process. Moreover, due to modal and spectral mismatches, coupling single photons from a nonlinear source to a quantum memory is generally not an efficient process. These make the successful generation of an entangled light-matter state highly probabilistic.
Entanglement between a single photon and a matter qubit is an indispensable resource for quantum repeater and quantum networks. With atomic ensembles, the entanglement creation probability is typically very low to inhibit high-order events. In this paper, we propose and experimentally realize a scheme that creates atom-photon entanglement with an intrinsic efficiency of 50%. We make use of Rydberg blockade to generate two collective excitations, lying in separate internal states. By introducing the momentum degree of freedom for the excitations, and interfering them via Raman coupling, we entangle the two excitations. Via retrieving one excitation, we create the entanglement between the polarization of a single photon and the momentum of the remaining atomic excitation, with a measured fidelity of 0.901(8). The retrieved optical field is verified to be genuine single photons. The realized entanglement may be employed to create entanglement between two distant nodes in a fully heralded way and with a much higher efficiency.
Summary form only given. Neutral atoms trapped inside an optical cavity provide an ideal platform for the implementation of quantum networks [1]. In such a network, nodes containing multiple atomic qubits are essential for the construction of a quantum repeater as they allow for entanglement swapping and thus the generation of entanglement between qubits over long distances. Here we will show the realisation of such a multi-qubit network node containing two87Rb atoms (see Figure. 1) in an optical cavity. Local entanglement between the two atoms is created with an experimental technique called quantum state carving. It relies on the reflection of weak coherent pulses from the cavity and subsequent measurement of the photons polarisation [2]. Postselection on the cases where the photon changed its linear polarisation by 90° after the reflection projects the atomic state onto an entangled state. This measurement technique is intrinsically probabilistic, but heralded. We will show the generation of all four maximally entangled atomic Bell states, a key resource for a plethora of quantum communication protocols. We will give an overview over the necessary experimental toolbox including the collective manipulation and readout of the two atomic qubits.